ATP synthesis after illumination wasalso enhanced by ATP.

The gamma subunit rotation plays a "catalytic" role as its rotation induces cyclic conformational changes in the beta subunit of the synthase. Can ATP synthesis occur without the gamma subunit by a mechanisms which involves a less proficient, but concerted set of cyclic change in beta subunit conformation? Apparently it can. Uchihashi et al have used high speed atomic force microscopy (AFM) to study the (alpha-beta)3 hexagon from the F1 subunit without the gamma subunit. They found that upon ATP hydrolysis, the beta subunits underwent conformational changes in the same counterclockwise rotary direction as when the gamma subunit was present.

Arsenic and its metabolites are primarily excreted in the urine.

9.1 and 9.2 Photosynthesis and ATP - 12 cards

Recent experiments (Wantanbe, 2010) using immobilized ATPase and magnetic tweezers have addressed the timing of substrate binding and product release when the enzyme is run in reverse (ATP hydrolysis). On rotation of the gamma subunit, the three binding sites change properties. In hydrolysis, ATP binds to the open site, and helps promote the 120 degree rotation. In the next step, ATP is hydrolyzed. In final step, products dissociate. Pi dissociation occurs last from the third site. Hence each of the 3 beta binding sites have different roles. One binds substrate, one performs catalysis, and third releases products. Assuming the synthesis pathway is the reverse of the ATPase reaction, the final release of Pi in ATP cleavage predicts that Pi binds first in the synthetic direction. This would preclude the binding of ATP next which is critical since its concentration during synthesis can be 10x higher than that of ADP. As Pi is bound first, only ADP, not ATP can bind next.

participating not only in ATP synthesis but ..

The actual amino acids involved in the mechanism of ATP synthesis/hydrolysis are still not clearly defined but Glu 190 on the beta subunit clearly acts as a general base. The figure below shows bound ADP and the proximity of Glu 188. Ala 158 is thought to move towards the active site after a conformational change, with the nonpolar methyl side chain displacing an adjacent water molecule which could leave as a product of ATP synthesis.

Steady-state kinetics of ADP-arsenate and ATP-synthesis …

At the level of the citric acid cycle, arsenic inhibits pyruvate dehydrogenase and by competing with phosphate it uncouples oxidative phosphorylation, thus inhibiting energy-linked reduction of NAD+, mitochondrial respiration, and ATP synthesis.

Interactions of arsenate, sulfate and phosphate with yeast ..

Whilst Wolfe-Simon et al. raise an intriguing hypothesis about possible alternative life chemistries, their supporting data is rather slim. Several scientists have already criticized the published data (, ), and our reaction upon reading the paper has been similar. The isolated strain grows faster on arsenate on its own than on phosphate on its own (Fig. 1A in ). However, growth rate under the condition which might be optimal for this strain, namely, high concentration of arsenate plus low phosphate concentration, has not been reported. DNA preparations from the newly isolated strain have been shown to contain arsenate, but these preparations also contain phosphate. Arsenate may be associated with the DNA, but no direct evidence has been provided to support the hypothesis that arsenate substitutes for phosphate in the DNA's base-linking diester bonds. In addition, whereas total nucleic acid preparation (DNA plus RNA samples analyzed for As and P content) from the cells grown on phosphate appear normal by gel-electrophoresis, the arsenate grown bacteria yielded only one band of discrete size (possibly chromosomal DNA) and no low molecular weight smear that usually corresponds to RNA (Fig. 2A in ). It could therefore be that growth under arsenate induces drastic change in the nucleic acid content and/or properties, as suggested by the authors. However, these nucleic acid samples extracted with standard methods, and examined in aqueous solution, are not necessarily relevant.

explain how arsenate can double oxidation and ..

but the wheel that provides the coupling to ATP synthesis is missing

Arsenic is known to induce the metal-binding protein metallothionein, which decreases the toxic effects of arsenic and other metals by binding them and making them biologically inactive, as well as acting as an antioxidant.

Regulation of Mitochondrial ATP Synthesis and Export …

Compared to the extensive literature on the hydrolysis of phosphate esters, data on the hydrolysis of arsenate esters is quite limited. It is known, however, that arsenate esters, including tri- and di-, and mono-esters, and pyro-arsenate, hydrolyze many orders of magnitude faster than the equivalent phosphate esters. The mechanisms of hydrolysis also differ, with arsenate esters hydrolyzing via mechanisms with a more associative character ( and references therein). High reactivity and the use of a more associative mechanism, result in a fundamental difference in the hydrolysis kinetics between phosphate and arsenate esters. Phospho-triesters are hydrolyzed ~105-fold faster than the corresponding diesters. The latter comprise the connecting bonds in DNA, and their remarkable stability is critical for genome integrity (). Arsenate-triesters are highly labile, the half-life of trimethyl-arsenate in water, at neutral pH and ambient temperature, is in the range of 0.02 sec. However, unlike the analogous phosphate esters, the resulting dimethyl ester of arsenate hydrolyzes even more rapidly (extrapolating from the data of ()). The 5'and 3'-ribosyl groups of DNA bases exhibit about a 10-fold lower rate of hydrolysis than methyl groups (diethyl-arsenate hydrolyzes ~5-fold slower than dimethyl-arsenate) (), but beyond that, no fundamental differences in reactivity between the dialkyl-arsenates used as a model and the diester bonds of DNA are expected. The corresponding arsenate esters of small metabolites such as NTPs and dNTPs (nucleotide, and deoxy-nucleotide triphosphates) are expected to be even more labile and will not sustain in aqueous environments.